1. Field of the Invention
This disclosure relates generally to processes for the production of elemental sulfur from sulfur dioxide, and more particularly to processes for the thermal reduction of sulfur dioxide, such as that from effluent streams, to elemental sulfur.
2. Description of the Related Art
Appreciable amounts of sulfur dioxide are found in many industrial gases vented into the atmosphere from plants involved in roasting, smelting and sintering sulfide ores, or gases from power plants burning high sulfur coal or fuel oils or other sulfurous ores or other industrial operations involved in the combustion of sulfur-bearing fuels, such as fuel oil. Air pollution as the result of sulfur dioxide emission in these gases presents not only health hazards, but also results in the loss of valuable sulfur values. One of the more difficult environmental problems facing industry is how to economically control SO2 emissions from these sources. One of the approaches is the recovery of sulfur dioxide from such gases and related waste streams, preferably in the form of elemental sulfur which may be reused to an economic advantage, especially as it may be stored and transported less expensively than other reduction products.
The reduction of sulfur dioxide, including its reduction to elemental sulfur, has been investigated extensively over the years. In fact, a number of processes are known for the reduction of sulfur-dioxide containing gases and gas streams to elemental sulfur. One proposed process, suggested in German patent application No. 2,365,116, describes a process for reducing sulfur dioxide-containing gases with relatively pure hydrogen at temperatures greater than 600° C. (1112° F.) without the use of a reduction catalyst. Due to this process requiring relatively pure hydrogen gas, this process is relatively expensive, and may not be appropriate for large-scale reduction processes.
Other approaches described in the art have utilized interesting methodologies, but methodologies which nonetheless are often not applicable to large-scale process situations, are cost inefficient, result in the formation of other, non-desirable products, or do not have high recovery and/or product quality values. For example, one reported process for the conversion of elemental sulfur to sulfur dioxide involves the reduction with a carbonaceous material such as coke, wood, or petroleum coke in a reaction zone containing a molten salt, such as sodium or potassium sulfide. Another approach to solving the problem at hand includes the reduction of sulfur dioxide (from an off-gas) to elemental sulfur via a process of converting the sulfur dioxide to sulfur by passing a sulfur dioxide-containing off-gas through a reactor containing a mixture of fresh and recycled coal. Finally, biological processes, such as those reported in U.S. Pat. No. 5,269,929, have been suggested, proposing the microbial reduction of sulfur dioxide to hydrogen sulfide using sulfate reducing bacteria in co-culture with anaerobic heterotrophs, such as the sulfate reducing bacteria belonging to the genera Desulfovibrio and the like, under anaerobic conditions. Reportedly, the SO2 converted by the microorganisms to H2S may then be reacted with some remaining SO2 in a Claus reactor in order to produced elemental sulfur, as desired. However, the applicability of this process to industrial-scale processes has not yet been shown.
From the above brief overview, it can be appreciated that numerous of the prior art desulfurization and/or sulfur dioxide-reducing processes have the distinct disadvantage of producing, at best, a byproduct having only nominal market value, and therefore having only a minor effect on the economic aspects of the sulfur dioxide reduction process. Accordingly, it would be desirable to have a sulfur dioxide reduction process which produces elemental sulfur as the major product, wherein such a process is economically and environmentally efficient.
This application for patent discloses processes for the thermal reduction of sulfur dioxide (SO2) to elemental sulfur in high recovery.